Cystatin C is a basic low molecular weight protein (isoelectric point pH 9.3) having a molecular weight of 13 kDa, and is continually produced in all of nucleated cells in the human body, while a certain amount is secreted extracellularly without being affected by environmental changes. Therefore, the concentration of cystatin C in blood becomes constant, and is not subject to the influence such as inflammation due to other diseases, or to the influence of age, gender, exercise, meal or the like. Furthermore, cystatin C does not form composites with other plasma proteins, but is filtered at the renal glomerulus and reabsorbed in the proximal renal tubule. Therefore, it is known that if the glomerular filtration rate (GFR) decreases, the blood concentration of cystatin C increases, and accordingly, cystatin C is attracting attention as an indicator of the GFR instead of creatinine. Furthermore, since cystatin C serves as a diagnostic indicator of early renal failure in test for renal function, cystatin C is widely useful also in the areas of ambulatory care, medical examination and the like.
In regard to the method for measuring cystatin C, there have been reported automated particle-enhanced immunoassay methods for human cystatin C (Non-Patent Documents 1 to 3, and Patent Document 1). For example, in the method described in Non-Patent Document 1, use is made of an anti-cystatin C polyclonal antibody having a strong agglutinating power, which is in the form of being bound to carrier particles at a weight ratio of 5%. However, as shown in Non-Patent Document 4, cystatin C belongs to the cystatin family in which the members are similar in structure, and cystatin C is differently localized in various types of human body fluids. Accordingly, there is a very high possibility that an accurate measurement value for cystatin C may not be obtained with a polyclonal antibody which has low specificity, depending on the type of the human body fluid (for example, saliva or lacrimal fluid). Meanwhile, Patent Document 1 describes information to the effect that generally an immunoagglutinate is more easily formed when a polyclonal antibody is used (page 20, lines 21 to 27). Furthermore, it is suggested to use an anti-cystatin C monoclonal antibody having high specificity. However, it is also described to the effect that in the case the target is a monomer and low molecular weight proteins such as cystatin C, better results are obtained by using a cocktail of many different monoclonal antibodies (page 21, lines 5 to 10), which is essentially equivalent to the use of a polyclonal antibody. Moreover, in order to improve the performance, it has been necessary to bind a markedly large amount of antibodies such that the binding amount of antibodies to the total weight of antibody-coated particles is greater than 5% by weight and equal to or less than 35% by weight.